This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application for Image Forming Apparatus For Half-Tone Recording And Shading Compensation And Half-Tone Image Forming Method earlier filed in the Korean Industrial Property Office on Apr. 10, 1996 and there duly assigned Serial No. 1080/1996.
1. Technical Field
The present invention generally relates to an image forming apparatus, and more particularly, relates to an image forming apparatus with an improved construction that simplifies shading compensation and half-tone recording. The present invention also relates to a half-tone image forming method.
2. Related Art
Image forming apparatuses such as digital photocopiers, facsimile machines and laser printers are systems in which graphic material in the form of an original copy is scanned and converted to an electrical signal to be reproduced on a recording medium. An image forming apparatus generally produces a half-tone image by scanning a document with a light source. An analog image signal, produced during the document scanning, is converted into a digital signal by an analog-to-digital converter (ADC). A reference voltage used in the ADC is fixed without consideration of respective output characteristics of image sensors. As a result, printed image with different half-tone may be reproduced, and at worst the gradation of the image becomes unclear.
One example of a half-tone image reproduction technique that seeks to reproduce a half-tone image with a proper density is disclosed in Korea Patent Application No. 93-5325 for Image Signal Processing Apparatus For Half-Tone Recording And Method Of Controlling The Image Signal Processing Apparatus assigned to the same assignee of the instant application. In this disclosure, a maximum amplitude of an image signal, produced by an image sensor during half-tone recording, is determined, and 90% to 95% of this maximum image signal (voltage) serves as a reference voltage (+Vref) in order to determine a dynamic range of the analog-to-digital converter ADC. This technique, however, requires the ability to control the reference voltage and incorporates a digital-to-analog converter DAC to convert a digital signal back to the corresponding analog signal. By eliminating the DAC, the image forming apparatus for half-tone recording could be simplified. Other half-tone image reproduction techniques are disclosed, for example, in U.S. Pat. No. 5,278,672 for Image Signal Processing Apparatus issued to Shimazaki, U.S. Pat. No. 5,309,254 for Image Processing Apparatus Capable Of Processing Half-Tone Image Data issued to Kuwabara et al., U.S. Pat. No. 5,519,509 for Image Processing Method Utilizing Error Diffusion Technique issued to Hikosaka et al., and U.S. Pat. No. 5,550,647 for Image Processing Apparatus For Converting A Multilevel Image Signal Into A Bi-Level Image Signal issued to Koike.
Conventional image forming apparatus also includes a shading correction system for compensating shading distortions due to differences in sensitivity between image sensing elements of an image sensor, light intensity distribution of the light source irradiating the document, and distortion of lens or the like in an optical system. Examples of shading correction system are disclosed in U.S. Pat. No. 5,084,772 for Shading Correction Method In Image Reading Device issued to Shimoyama, U.S. Pat. No. 5,099,341 for Image Reading Apparatus With Improved Shading Correction issued to Nosaki et al., U.S. Pat. No. 5,253,083 for Image Reading Apparatus Having Improved Shading Correction issued to Hirota, U.S. Pat. No. 5,317,421 for White Reference Data Generating Unit Applied To Shading Correction System issued to Ito, and U.S. Pat. No. 5,422,740 for Image Forming Apparatus Which Effects Output Gray Level Correction issued to Fujimoto et al. Generally, in order to compensate for shading distortions, a shading factor for every scanned pixel is assessed during a dummy scanning operation. Shading distortions are compensated by multiplying each pixel converted into a digital signal during a real scanning operation by respective shading factors. For shading compensation, the conventional technique requires the step of finding a maximum brightness value among the dummy-scanned pixels. If this step can be avoided, as I have observed however, the shading compensation will be less complicated and will require less time to complete.
Accordingly, it is therefore an object of the present invention to provide an improved image forming apparatus and process of half-tone recording and shading compensation of an image.
It is also an object to provide an image forming apparatus with a simplified construction for performing both the half-tone recording and shading compensation of an image.
It is another object to provide a method of simplifying the construction of an image forming apparatus and its operational processes required for half-tone recording and shading compensation of an image.
It is still another object to provide an image forming apparatus and process for performing the half-tone recording and shading compensation faster than possible with the present art.
These and other objects of the present invention can be achieved by an image forming apparatus with an image sensor for scanning a document to produce an electric signal representative of an image from the document which includes a converter for converting the electric signal to produce image data with a predetermined number of bits; a shading memory unit for storing said image data on a pixel by pixel basis from a least significant address to a most significant address scanned during a first mode operation, and for storing shading factors each corresponding to each pixel from the image data; a controller for sequentially reading said image data stored from the least significant address to the most significant address of the shading memory unit during the first mode of operation, calculating each shading factor for each pixel by dividing a preset maximum brightness value by the image data sequentially read from the shading memory unit, recording the shading factors in the shading memory unit, and for outputting the shading factors stored in the shading memory unit corresponding to each pixel of the image data produced by the converter during a second mode of operation; a shading compensation unit for multiplying each pixel of the image data produced from the converter by a corresponding one of the shading factors during said second mode of operation to produce shading-compensated image data; and a half-tone processing unit for producing a half-tone image from the shading-compensated image data produced from the shading compensating unit.